Blizzard have stated on numerous occasions that the Diablo 3 beta test is predominantly to test technical aspects of Diablo 3’s delivery, namely the downloader and server infrastructure. That doesn’t fully explain why the population of testers is still very small however but in answer to that question Bashiok clarified why it’s low and at which points it’s likely to expand.
We have actual total population numbers at any given time, though, and we’re well aware of it and monitoring how many people we want to let in and when.
As the test is mainly for the downloader, patcher, launcher, various behind-the-scenes services, and server hardware, there are specific updates or milestones for these we’re tracking and they will likely determine more invites. For instance, if we have a big update coming for the installer there’s no reason to let people in now, we’d wait until the new installer is available, invite a bunch of people, and see how it does. Similarly if we’re expecting new server hardware we wouldn’t be too interested in stress testing what we’re using right now, we’d wait until we got the new hardware before we cared to let too many more people in.
Source: Blizzard US.